The term “5′-nucleotides” generically refers to a collective term for flavor-enhancing nucleotides including 5′-adenylic acid, 5′-guanylic acid, 5′-inosinic acid, and 5′-xanthylic acid, which is sometimes also referred to as nucleic acid-based seasonings. 5′-nucleotides have a strong flavor-enhancing effect even when used alone. It is known that the addition of 5′-nucleotides into a soy sauce containing L-glutamic acid as an inherent flavor-enhancing ingredient significantly improves and enriches the flavor through a synergistic effect.
However, since soy sauces contain a large amount of phosphatases derived from various microorganisms, even when such flavor-enhancing nucleotides are added to a soy sauce, the nucleotides are dephosphorylated by the action of the phosphatases, and decomposed into nucleosides without flavor-enhancing property.
For example, a raw soy sauce before pasteurization is one that exhibits a particularly strong phosphatase activity, and it is known that flavor-enhancing nucleotides added to the raw soy sauce are substantially decomposed and lost in a day after the addition, resulting in complete loss of the flavor-enhancing property of these nucleotides (Patent Document 1). A raw soy sauce has a considerably strong phosphatase activity, but the soy sauce pasteurized (at 80° C.) on a usual factory scale is also known to have a residual phosphatase activity of about 10 to 25% that of the raw soy sauce.
Therefore, it is necessary to inactivate the phosphatase activity to stably maintain the 5′-nucleotides added into a soy sauce (Patent Document 2).
In conventional methods for producing 5′-nucleotide-containing soy sauces, the phosphatase activity of a soy sauce produced by a common method is inactivated before the addition of 5′-nucleotides. To produce and accumulate 5′-nucleotides in a soy sauce moromi mash (i.e., unrefined soy sauce) juice without the addition of 5′-nucleotides has been considered to be impossible, and hence has not been effected.
On the other hand, methods for producing soy sauces containing a nucleic acid as a flavor-enhancing ingredient are described in some documents. Patent Document 3 discloses a method for producing a rich soy sauce characterized in that a step of preparing moromi mash for naturally brewed soy sauce includes the use of a highly flavor-enhancing liquid obtained by adding water to fermented lees of sake (a Japanese rice wine) or the like and autolyzing the mixture at 50 to 60° C. for 1 to 2 weeks, or a compressed liquid thereof, as shikomi water (water used to prepare moromi mash) added to a raw material after the koji making process. Moreover, Patent Document 4 discloses a method for preparing a liquid seasoning containing various amino acids, nucleic acid-based flavor-enhancing substances, and natural potassium. In this method, distillation residue (or its concentrate) of singly distilled shochu (a Japanese distilled spirit) made from sweet potatoes is mixed with soy sauce koji and aqueous common salt solution, the mixture is placed in a container for fermentation and aging, and the resulting mixture is filtered. However, both of these methods require special ingredients (i.e., sake lees or distillation residue of shochu). Therefore, the resulting soy sauces have the drawback that they taste considerably different from typical brewed soy sauces.
Moreover, the present inventors conducted re-examination of the inventions described in Patent Documents 3 and 4, and found that the resulting soy sauces of both of the inventions are devoid of 5′-nucleotides. This supports the fact that since soy sauces contain a large amount of phosphatases derived from various microorganisms, even when such flavor-enhancing nucleotides are added to a soy sauce, the flavor-enhancing nucleotides are dephosphorylated by the action of the phosphatases, and decomposed into nucleosides without flavor-enhancing property.
Patent Document 1: JP Patent Publication (Kokoku) No. 53-33661 B (1978)
Patent Document 2: JP Patent Publication (Kokoku) No. 39-27496 B (1964)
Patent Document 3: JP Patent Publication (Kokai) No. 54-84097 A (1979)
Patent Document 4: JP Patent Publication (Kokoku) No. 4-62707 B (1992)